Various organic membranes and inorganic membranes have conventionally been proposed as separation membranes. However, organic membranes have low solvent resistance and heat resistance, although they are inexpensive and excellent in moldability. As opposed to organic membranes, inorganic membranes, such as ceramic membranes, have excellent solvent resistance and heat resistance; however, they have problems of high cost and difficulty in molding.
Accordingly, carbon membranes, which are inorganic membranes, but have excellent moldability and are inexpensive, have recently attracted attention. Hollow fiber carbon membranes have pores of a size that allows gas separation, and exhibit excellent gas separation performance among various inorganic membranes. Further, hollow fiber carbon membranes can be used in an environment for which heat resistance against a temperature as high as about 70 to 150° C., at which organic membranes cannot be used, chemical resistance, and solvent resistance are required. Accordingly, the practical use of hollow fiber carbon membranes is highly expected. Moreover, hollow fiber membranes have advantages of excellent pressure resistance, a large membrane area per unit volume, and capability of producing compact separation membrane modules.
Conventionally proposed hollow fiber carbon membranes are those produced, for example, using a resin obtained by sulfonating polyphenylene oxide (Patent Documents 1 and 2), and using aromatic polyimide (Patent Document 3), as a raw material.
However, sulfonated polyphenylene oxide itself is not a versatile material, and therefore requires a synthesis process to sulfonate polyphenylene oxide. On the other hand, the synthesis of aromatic polyimide requires a reaction in an organic solvent; however, since it is difficult to ensure the solubility in the organic solvent, a special production method is necessary. Thus, carbon membranes produced using sulfonated polyphenylene oxide or aromatic polyimide as a raw material have problems of high membrane cost, because raw materials are expensive, and the preparation of raw materials and the membrane-forming process are complicated.
In contrast, a carbon membrane produced using inexpensive polyphenylene oxide as a raw material is also proposed (Patent Document 4). However, separation properties are low only with polyphenylene oxide; therefore, ensuring separation properties requires a complicated structure in which a sulfonated polyphenylene oxide resin is laminated on a polyphenylene oxide membrane, followed by calcination treatment, and the production process is complicated. Accordingly, there is a problem of high cost, despite the use of the inexpensive raw material.
Carbon membranes produced using any raw material generally require two-step heating comprising, in this order, an “infusibilization treatment” in which a hollow fiber is spun, cut into a predetermined length, then inserted in a tube made of PFA resin, etc., and heated at about 250 to 350° C. in the air; and a “carbonization treatment” in which heating is performed at about 600 to 800° C. in an inert atmosphere or under vacuum.
Therefore, in order to produce carbon hollow fiber membranes with excellent cost performance, there is a demand for a production method in which a hollow fiber is spun using inexpensive materials, and without adopting complicated processes, a two-step heating process comprising an infusibilization step and a carbonization step is performed as a main process.
Furthermore, there is an increasing demand for carbon membranes with still higher functionality having a high permeability rate for helium, hydrogen, steam, etc., and high ability to separate these gases.